Launching a satellite is an expensive and time consuming business, but I have often felt that the most nerve-wrecking part of the process must be after the launch. If anything major malfunctions on the spacecraft, it's highly unlikely that you will ever be able to do anything about it as repairs, faulty part replacements and refueling are practically impossible, or at least they have been until this experimental spacecraft was launched for testing today:

Orbital Express: Prototype Satellites Primed for In-Flight Service

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Updated 2021 : note that the above post is out of date as it was originally published in 2007, but is left here for archival purposes.